NR TI 027 ISSUE 3 PDF

This TI applies to small T/J sets (not exceeding 120V or 5A output) that supply external circuits, and DC track circuit feed sets, except where alternative protection already exists. It does not apply to constant voltage battery chargers, used to charge modern sealed batteries. For retrospective fitment, this TI includes only T/J sets having integral terminations using studs and nuts, or screw terminations, to which the Varistor may be fitted.

Purpose

During lightning storms, damage to electrical signalling equipment is not uncommon. Although little can be done to protect against a direct strike, experience has shown that the silicon bridge rectifiers in nearby T/J and DC track circuit feed sets are also damaged. Improved survival rates can be achieved by fitting a voltage-dependent device, such as a Varistor, across the rectifier output to absorb voltage spikes and keep them below the rectifier's peak inverse voltage rating. This TI requires new installations to be fitted and provides a process for maintenance to fit existing equipment.